U.S. patent number 4,384,925 [Application Number 06/200,107] was granted by the patent office on 1983-05-24 for gas sensing unit with automatic calibration method.
This patent grant is currently assigned to Becton Dickinson and Company. Invention is credited to Lawrence Spritzer, Joseph R. Stetter, Solomon Zaromb.
United States Patent |
4,384,925 |
Stetter , et al. |
May 24, 1983 |
Gas sensing unit with automatic calibration method
Abstract
Methods and apparatus are provided for sensing gases in the
environment wherein electrochemical sensing procedures are utilized
not only for monitoring ambient continuously for the presence of
such gases; but also, for the periodic automatic recalibration and
self-adjustment of the sensing instrument, as required, to
accommodate changing conditions with time. Such accommodation
includes not only changes in the instrument itself, but also
changes in the environment affecting the accuracy of the monitoring
function. The instrument is connected with a microprocessor, or
other information storage or retrieval instrumentation which
controls the periodic recalibration by measuring, separately from
the monitoring function, the electrochemical response to a sample
of the gas being monitored, and by adjusting the subsequent
instrument readings to reflect the recalibration. During this
recalibration procedure, the instrument can be adjusted to zero
reading for accommodating drift, as will be understood. The
microprocessor may be adjusted, also, to cause the automatic
recalibration of the sensor instrument at preset intervals, as
required at the location where it is being used. Thus, the
instrument of the invention has the ability to perform a
calibration without the need for an operator or calibration gas.
The requirement is that the instrument be sensing the gas of
interest at the time of calibration.
Inventors: |
Stetter; Joseph R. (Naperville,
IL), Spritzer; Lawrence (Peekskill, NY), Zaromb;
Solomon (Newark, NJ) |
Assignee: |
Becton Dickinson and Company
(Paramus, NJ)
|
Family
ID: |
22740362 |
Appl.
No.: |
06/200,107 |
Filed: |
October 24, 1980 |
Current U.S.
Class: |
205/785.5;
204/401; 204/406; 204/409; 204/411; 340/632; 422/67; 422/98;
702/104; 702/24; 73/1.07 |
Current CPC
Class: |
G01N
33/0006 (20130101) |
Current International
Class: |
G01N
33/00 (20060101); G01N 027/42 () |
Field of
Search: |
;364/571,497,500
;204/195R ;73/1G,1R ;340/632 ;422/67,98 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Buchanan, E. B., Jr., Talanta, vol. 27, pp. 947-954, Nov.
1980..
|
Primary Examiner: Kaplan; G. L.
Claims
What is claimed is:
1. Self-calibrating electrochemical sensor apparatus for
continuously monitoring and detecting an electrochemically active
gas comprising
(a) a housing;
(b) an aqueous electrolyte contained in said housing;
(c) a sensing electrode substantially continuously in contact with
said electrolyte and the ambient fluid around said apparatus to be
monitored which may contain an electrochemically active gas;
(d) a counterelectrode in contact with said electrolyte;
the improvement characterized by
(e) automatic control means connected to said apparatus;
(f) means in said automatic control means for controlling and
measuring the flow of ambient fluid to said apparatus;
(g) said controlling and measuring means including means for
segregating a specific sample of ambient fluid, said segregated
sample being of unknown concentration of any electrochemically
active gases contained therein;
(h) said segregating means including
(1) a first reactant chamber in contact the said sensing electrode
and having a first ambient fluid inlet and a first ambient fluid
outlet;
(2) a second reactant chamber in contact with a second sensing
electrode having a second ambient fluid inlet and a second ambient
fluid outlet;
(3) said first and said second inlets and outlets being connected
in series;
(4) a pump and valving system for causing flow of said segregated
sample of fluid through said first chamber and then into said
second chamber, and then causing reversed flow whereby the said
segregated sample of fluid is caused to flow first through said
second chamber and then into said first chamber;
(i) a flow meter in said controlling and measuring means for
measuring the rate of flow past said sensing electrode of a sample
of an ambient fluid being monitored;
(j) means in said automatic control means for measuring the signal
from said sensor apparatus in response to said segregated
sample;
(k) means in said automatic control means for establishing the
concentration of an active gas in said segregated sample;
(l) means in said automatic control means for storing the values of
said signal and of said concentration; and
(m) means in said automatic control means for adjusting the
calibration of said sensor apparatus according to the values of
said storing means during said continuous monitoring and detecting
thereof.
2. The apparatus of claim 1, further characterized by
(a) a reference electrode in contact with said electrolyte; and
(b) electrical communication means between said reference electrode
and said sensing electrode for selectively establishing a fixed
potential on said sensing electrode.
3. The apparatus of claim 1, further characterized by
(a) said automatic control means is a microprocessor.
4. A method for the automatic recalibration of a gas sensing and
monitoring unit which is in continuous monitoring and sensing
operation, characterized by the steps of
(a) connecting a gas sensor unit to an automatic control;
(b) placing said gas sensor unit in an area to be monitored;
(c) establishing a first and a second gas sensing area in said gas
sensing unit;
(d) continuously monitoring and sensing the ambient gas in the area
established by said placing step;
(e) periodically segregating a sample of ambient gas being
continuously monitored and sensed;
(f) flowing in a first flowing step said segregated sample through
said first gas sensing area and then into said second gas sensing
area;
(g) flowing in a second flowing step said segregated sample through
said second gas sensing area and then into said first gas sensing
area;
(h) periodically measuring the signal from said sensor unit in
response to the gas present in said sample of the ambient by
controlling and measuring the flow of said sample in said first and
second flowing steps;
(i) establishing the concentration of any sensed gas in said sample
from said controlling and measuring step;
(j) inserting the values obtained from said signal controlling and
measuring and concentration establishing steps into said automatic
control; and
(k) converting the values from said inserting step into the
continuous sensing and monitoring readings of said sensor unit.
5. The method of claim 4, further characterized by the steps of
(a) preselecting a periodic interval for recalibrating said gas
sensor unit; and
(b) entering said preselected period into said automatic
control.
6. The method of claim 4, further characterized by
(a) said concentration establishing step is carried out by
segregating and measuring coulometrically a preselected quantity of
the gas being monitored.
7. The method of claim 4, further characterized by
(a) providing an electrochemical gas generating cell;
(b) connecting said gas generating cell to said gas sensor unit;
and
(c) supplying gas of unknown concentration from said gas generating
cell for said concentration establishing, signal measuring, and
converting steps.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to gas detector units for
monitoring noxious gases in the environment in the area where the
units are placed so that persons involved in the environment where
the instruments are located may be warned of the presence of a too
high level of a noxious gas. More particularly, this invention
relates to such a gas sensing unit which has incorporated therein
provisions for recalibration of the instrument automatically at
periodic intervals without the need for specific recalibration by
an individual. That is, the instrument is controlled by a connected
microprocessor-type instrument which controls the periodic
recalibration of the gas sensing unit. By proper control through
the microprocessor, various valve connections may be operated to
cause the gas sensing unit to separate from its usual monitoring
function and to calibrate through specific procedures, so as to
accommodate changes in the instrument over a period of time and
changes in the environment where it is functioning. The
recalibration is incorporated into the microprocessor so that
subsequent monitoring readings are automatically adjusted to
reflect the recalibration. This invention is an improvement over
the inventions described in U.S. Pat. Nos. 3,992,267, issued Nov.
16, 1976; 3,824,167, issued July 16, 1974; 3,776,832 issued Dec. 4,
1973 and 3,909,386, issued Sept. 30, 1975. Each of these patents is
incorporated by reference in its entirety herein.
With ever increasing concern about pollution of our environment and
our increasingly sophisticated knowledge with respect to the
presence of polluting materials in the environment, attempts have
been made to develop systems which will protect us by warning of
increases in the concentration of certain substances in the ambient
to a level which is toxic and/or otherwise dangerous to our
existence. One such device which has been developed in recent years
is a gas sensor operated through an electrochemical cell for
sensing the presence of such gases adjacent a work area such as,
for example, a mine shaft. As will be appreciated, it is important
that such sensors continue to operate over a period of time so that
certain enclosed areas are protected where certain levels of
concentration of gases may cause death if exposure is for a
specific period of time. It is important, also, from a manpower
standpoint that the instruments need not be continuously attended
to because of any rapid deterioration of the sensing capacity
thereof. However, electrochemical gas sensors, for example, are
subject to certain limitations over a period of time merely because
of the chemical nature in which they operate, in the sense that the
sensing capacity changes with environmental conditions.
Thus, there is a need with respect to present day instruments for
maintenance at periodic intervals, and especially recalibration
thereof. That is, such instruments change their performance
characteristics over a period of time and it is necessary that they
be recalibrated so as to restore accuracy. For example, presently
carbon monoxide monitors which are placed around a hazardous area
such as a blast furnace for monitoring, for example, 100-600 ppm CO
are being calibrated once a month by a maintenance routine which
requires an operator to visit each instrument installed for this
purpose. The operator carries a certified calibration gas mixture
of carbon monoxide and air which is typically 250 parts per million
carbon monoxide/air. The certified span gas mixture is injected
into the instrument to verify its response, and an adjustment is
made in the instrument sensitivity, if required.
STATEMENT OF THE INVENTION
With this invention, by contrast, vastly improved sensor
performance characteristics are achieved in the sense of gaining
repeatability of the signal magnitude and zero selectivity and
long-term stability of the system's performance by connecting the
sensor to a conventional microprocessor or other programmable
instrumentation which is programmed to monitor the sensor
performance and, depending upon the program placed in the
microprocessor, determines on a set periodic basis when the sensor
is to be recalibrated. At that point, the sensor cell may be
disconnected from the monitoring function and a set programmed
sequential recalibration procedure takes place controlled by the
microprocessor connected to the sensor unit and its associated
valving structures. The recalibration procedure comprises measuring
the sensor signal in response to a specific gas sample,
establishing the concentration of the sensed gas in said sample,
and storing in the microprocessor the values of said signal and of
said concentration. This sequence is carried out automatically by
the pre-programmed microprocessor. Based on said values, the
microprocessor recalibrates the sensor to incorporate the new
readings and extrapolates those new readings into the subsequent
monitoring readings being taken by the sensor.
Also, as mentioned above, this recalibration procedure may be
arranged to be carried out at preset intervals of, perhaps, once a
week or once a month, depending upon the gas being monitored and
the environment where it is being monitored.
Of course, the methods and apparatus of the invention here are not
limited to the specific arrangement wherein the gas sensor cell is
stopped from its monitoring function during the recalibration
procedures. Arrangements may be made, in accordance with this
invention, to include in a sensing unit at a particular location, a
separate gas sensing cell which operates chiefly to carry out the
recalibration function, while the main cell is maintaining the
monitoring function during this period of time. In this way, the
monitoring function is never interrupted and this is particularly
important in instances where the location involved is exposed to a
highly toxic gas. Again, with such an arrangement, a microprocessor
is coordinated with both cells to institute the recalibration
function of one sensor cell while maintaining the other sensor cell
in its monitoring function. Subsequent to the recalibration of one
sensor, the microprocessor incorporates and extrapolates the
calibrated information into the monitoring function of the other
sensor. As will be apparent to practitioners in the art, other
arrangements may be made, in accordance with this invention, for
the automatic recalibration procedure for determining
coulometrically the current conditions for sensing a specific gas,
according to the location and requirements of the instrument
involved.
The invention also includes provisions for supplying, again
automatically, a sample of gas for the periodic automatic
recalibration of the instrument. That is, in certain instances, the
concentration of monitored gas present in the ambient may not be
sufficient to provide the appropriate readings for recalibration
purposes. Under these circumstances, a separate unit may be
interconnected with the sensor and the microprocessor for the
purposes of supplying on a periodic basis as needed an adequate
sample of the gas being monitored from a specific source so as to
provide a sufficient quantity of the gas for the calibration
procedure. For example, in a gas sensing situation where carbon
monoxide is being monitored, frequently a low carbon monoxide
content is present in ambient. Such a low carbon monoxide content
may not be sufficient to carry out the automatic calibration
procedures with the desired degree of accuracy. Thus, carbon
monoxide may be produced in a separate attached instrument wherein
the anodic electrolysis of water produces hydrogen ions which react
with a sodium formate solution to produce the required carbon
monoxide for the automatic recalibration procedure.
Thus, with this invention, a supply of a sample gas may be provided
or generated so that the proper recalibration of the instrument
takes place even though, at any one time, there may not be a
sufficient concentration of the gas being monitored to carry out
the automatic recalibration of the instrument as required.
With the foregoing and additional objects in view, this invention
will now be described in more detail, and other objects and
advantages thereof will be apparent from the following description,
the accompanying drawings, and the appended claims.
IN THE DRAWINGS
FIG. 1 is a diagramatic view in block form of a gas sensing unit
illustrating the invention;
FIG. 2 is a cross-sectional view of an electrochemical cell for a
gas sensing unit of the invention;
FIG. 3 is a schematic block diagram of a gas flow circuit for
periodic recalibration of the cell illustrated in FIG. 1, according
to one embodiment of the invention;
FIG. 4 is a diagramatic illustration in block form of an
alternative arrangement of apparatus illustrating the invention
herein wherein a separate unit is connected for providing a sample
of gas for carrying out the automatic recalibration of the sensing
unit;
FIG. 5 is a diagramatic view similar to that in FIG. 4 showing yet
another arrangement wherein two sensing cells may be incorporated
in series with a flow meter and electrically actuated valves are
provided for reversing flow between the two cells for carrying out
the automatic recalibration of the sensing unit; and
FIG. 6 is a block diagram of still another arrangement wherein
automatic recalibration is achieved with a single sensing cell
using at least two different gas flow rates.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawings, in which like reference characters refer
to like parts throughout several views thereof, a gas detecting
device for the measurement of noxious gases is positioned within a
housing 10. The device includes a sample intake means 11 in direct
communication with line 12 for feeding the gas being monitored to
the sensor 15 (electrochemical cell) which, in turn, is connected
with a flow meter 17. Gas flowing through sensor 15 exits through
exhaust outlet 20. Valve 19, positioned between flow meter 17 and
outlet 20 may be provided to control the flow of gas exiting from
the unit. The sensor is provided with a potentiostat 24 for
maintaining a fixed relative potential between the sensing
electrode and the reference electrode of sensor 15, and with a
voltmeter 26. The potentiostat is hooked up to an electronic
circuit described in the afore-mentioned patents.
One form of electrochemical sensor which may be used in accordance
with this invention is shown in FIG. 2 and includes a
counterelectrode 25, a sensing electrode 27 and a third or
reference electrode 29, all positioned within a housing 31. In the
embodiment shown in FIG. 2, the counterelectrode, sensing electrode
and third electrode are in contact with a free-flowing aqueous
electrolyte 33. The nature of the electrolyte will be chosen, as
will be understood by practitioners in the art, according to the
gas being sensed in a particular location. Adjacent sensing
electrode 27 is a reactant chamber 35 having an inlet 36 and an
outlet 38. Counterelectrode 25 is in direct communication with
atmospheric air. Both the sensing and counterelectrodes are
lightweight electrodes comprising a hydrophobic plastic substrate
(polytetrafluoroethylene) 40, 42, respectively, in a direct contact
with reactant chamber 35 in the case of sensing electrode 27, and
with the ambient environment in the case of counterelectrode 25.
Catalytic film layers 44, 46, respectively, have been arranged on
the hydrophobic plastic substrate by various procedures such as,
for example, vacuum vapor deposition to a desired thickness as
required. These catalytic film layers are in contact with the
electrolyte of the cell, as will be understood. The reference
electrode 29 may be a porous electrode comprised of a
polytetrafluoroethylene substrate with an appropriate metallic
catalytic film disposed thereon. A fixed potential of within the
range of between about 0.4 and 1.5 volts relative to the standard
hydrogen electrode depending upon the gas to be detected, is
maintained on the sensing electrode by means of the reference
electrode through the potentiostat 24. For further details
concerning the arrangement of the imposed fixed potential on the
sensing electrode, references are made to the above-noted
patents.
In one embodiment of the invention illustrated in the block diagram
of FIG. 3, a microprocessor 80 records and stores the signals
(i.e., voltage or current outputs) from two electrochemical sensors
81 and 82, and from a mass flowmeter 83, and also controls three
electrically actuated valves 84, 85, and 86. (The term
"microprocessor" as used herein is for the purpose of designating
any form of instrumentation including any simple electronic
instrumentation which will function to store information and
control in a simplified manner the opening and closing of a number
of solenoid valves, for example, in sequence to control the valving
functions of the apparatus herein. One representative such
instrument is Model Number HP9815A manufactured by the Hewlett
Packard Company which is a programmable calculator, with
appropriate connections for accessory instruments.) Sensor 82
serves during the usual continuous monitoring purposes, while
sensor 81 remains idle most of the time and takes over the
monitoring function only during the relatively brief periods during
which sensor 82 is being calibrated. When sensor 81 is in the idle
state, any access of air thereto is cut off by valves 85 and 86.
Ambient air is then pulled by pump 93 through inlet 92 and valve 85
into sensor 82, and then through valve 86 and flowmeter 83. To
provide approximately constant air flow through sensor 82, a
by-pass throttle 94 is also connected to the flow circuit by valve
86 whenever circulation through sensor 81 is shut off.
Any small amounts of the measured pollutant trapped in the lines 90
and 91 connecting to sensor 81 and in its reactant chamber 35 (FIG.
2) at the start of the idle state are gradually consumed at its
sensing electrode 27 (FIG. 2), and become insignificant, usually
within the first 10 minutes, so that the signal from sensor 81
during most if its idle period yields the "zero" or "background"
signal S.sub.1 .degree. of this sensor. To calibrate sensor 81, the
normally closed valve 84 is first caused by microprocessor 80 to be
opened for a predetermined short period of time so as to permit a
sample of calibrating gas 88 from a compressed gas bottle 87 to
enter an expansible bag 89. The capacity of bag 89 may be between 1
and 10 liters, for example. Gas 88 contains a previously calibrated
concentration of the noxious pollutant detected by sensors 81 and
82 and the value of this calibrated concentration is stored in the
memory of processor 80. Once bag 89 has been filled with gas 88,
valve 84 is closed, and valve 86 causes throttle 94 to be closed
off and the outlet line 91 of sensor 81 to be connected to the line
95 leading to flowmeter 83 and pump 93, while valve 85 causes the
inlet 90 of sensor 81 to be connected to the line 96 leading to bag
89. The steady-state signal from sensor 81 can then be translated
by the micoprocessor into a calibrated response constant k.sub.1
for sensor 81. Given the background signal S.sub.1 .degree. and the
response constant k.sub.1, sensor 81 is ready to take over the
monitoring function while sensor 82 is being calibrated.
Bag 89 is completely emptied following each calibration to prevent
a gradual excessive gas build-up therein during repeated
calibrations. Full evacuation of bag 89 is recognized by the
microprocessor through a sharp decrease in the signals from the
calibrated sensor and from flowmeter 83, whereupon valve 85 is
actuated to disconnect the calibrated sensor from line 96, and to
connect it to the ambient air inlet 92. The calibrated sensor
thereupon assumes the monitoring function.
To calibrate sensor 82, valves 85 and 86 are actuated to shut off
circulation to sensor 82 and to connect throttle 94 to line 95,
thereby bringing sensor 82 into the idle state. After about 10
minutes or more, the background signal S.sub.2 .degree. of sensor
82 is recorded and stored in the microprocessor memory, and the
afore-described calibration procedure may be repeated for sensor 82
to yield the corresponding response constant k.sub.2. Sensor 82 is
then caused to resume its monitoring function, while sensor 81 is
returned to its idle state.
Of course, as will be understood by practitioners in the art, it is
also possible to have sensors 81 and 82 share the monitoring
functions for approximately equal time intervals.
As further illustrative of the invention herein, one may note FIG.
4 in which a diagramatic illustration in block form of another
arrangement of apparatus illustrating the invention is shown. In
the form of instrumentation shown here, an intermittent scheme of
recalibration of a gas sensing instrument is illustrated. Thus,
ambient air from source 64 is drawn by pump 13 into the system with
the flow through the various lines being controlled by valves 48,
50, 52 and 54, with the functioning of the valves being under the
control of a microprocessor 66.
In this case, valves 50, 52, 54 and 48 may be set to allow
circulation of ambient air to be monitored through lines 69, 70, 12
and into sensor 15 contained in the instrument 10, and thereafter
through line 20 to exit through valve 48. At one point, valves 54
and 48 are closed in order to segregate the air being sensed in
sensor cell 15 in unit 10. In this connection, unit 10 may contain
two sensor cells, with one for the purposes of a continued
monitoring of ambient air from source 64 under the direction of the
pump 13, while the second separate cell within housing 10 is for
the specific purpose of the coulometric measurement of a segregated
specific quantity of the air to be measured by the instrument. If
such an arrangement is made, there will be a separate valving
arrangement under the operation of the microprocessor 66 for
segregating the coulometric cell for this separate measurement.
In any case, once the segregation procedure has taken place under
the direction of the microprocessor, the microprocessor computes
the number of coulombs generated from the segregated volume of air
being measured, and the resulting reading in parts per million of,
for example, carbon monoxide is determined, and thereafter
calibrated into the readings of the microprocessor. Further, for a
blank determination of the coulombs generated after CO has been
removed from the air sample, valve 50 may be caused, along with
valves 52, 54 and 48, to shunt air through the zero air filter 58,
valve 48, the sensor cell in housing 10, and through valve 54.
The difference between the numbers of coulombs generated with and
without the measured pollutants present in the sampled air is an
absolute measure of the concentration of said pollutant within the
segregated sample, provided that the volume of the sample and the
ambient pressure and temperature are also given, and assuming
approximately 100% faradaic efficiency for the reaction of the
pollutant at electrode 27 (FIG. 2). The sample volume can be
established during manufacture of instrument 10 and should not
significantly change with time. The pressure and temperature can be
measured by appropriate transducers and stored in the
microprocessor memory. By comparing the value of the
coulometrically determined concentration with the sensor signal
just before segregation of the sample, the microprocessor can
compute the sensor response constant, as in the embodiment of FIG.
3.
As a further feature of the invention herein and in those instances
wherein there is not a sufficient concentration of the gas being
monitored in ambient for the automatic recalibration of the
instrument to take place, electrochemical generation of the sensed
gas may be carried out in an electrochemical generation cell 56.
For this purpose, ambient air is passed through cell 56 under the
direction of valves 52 and 54 so as to supply through line 12 air
with sufficient quantity in parts per million of carbon monoxide
for the cell in housing 10 to effectively measure for the purposes
of automatic recalibration. Again, after sufficient air with an
elevated carbon monoxide content has passed into the sensor cell in
housing 10, valves 54 and 48 operate to close off the cell for the
recalibration purpose. As will be understood, the electrochemical
generation cell 56 operates from a current source 57.
As stated above, various gases to be measured may be developed
electrochemically for the purposes of automatic recalibration of a
sensing system, in accordance with this invention, in those
instances where there is not a sufficient concentration of the gas
being sensed ordinarily in the ambient air being monitored. For
example, formaldehyde may be generated by injecting dichloromethane
in an aqueous alkaline solution. Alternatively, formaldehyde may be
generated electrochemically by cathodic reduction of formic acid at
a lead electrode at a low current density, or by cathodic reduction
of oxalic acid in aqueous sulfuric acid solution at a lead, carbon
or mercury electrode. Ethylene oxide may be formed by the reaction
of ethylene chlorohydrin with an alkaline solution. Alternatively,
it may be generated by electrochemical oxidation of ethylene at a
zinc/zinc oxide anode in an aqueous sodium benzoate, tungsten
oxide, or potassium carbonate solution. Hydrazine may be generated
by the reaction of urea with sodium hypochlorite.
As further illustrative of the invention herein, one may note FIG.
5 which shows a further form of instrumentation for carrying out
the invention here. In this case, two sensors are incorporated in
housings 10a and 10b and the currents from both are monitored,
while flow is through both sensors in series in one direction. At
some point in time, the flow is reversed and the gas being sensed
passes through the two sensors in the opposite direction. In this
connection, a mass flowmeter 62a measures the flow through the two
sensor units as controlled by the microprocessor 66 through an
appropriate electrically activated valving arrangement, as will be
understood.
As discussed previously above, the invention contemplates the use
of two sensors operating under the control of the microprocessor 66
in which one sensor is used continuously as the monitoring sensor
for monitoring the ambient where the unit is operating. A second
sensor is segregated from this monitoring function and used solely
for the purpose of the automatic recalibration of the instrument on
a periodic basis, as desired. Such an arrangement may include, in
accordance herewith, a two-sensor system wherein a single sensor
unit is provided with a five electrode system. Under this
arrangement, there are two sensing or working electrodes, each with
an independent gas exposure chamber, and two counterelectrodes with
one each connected to operate with one of the sensing electrodes,
and with a single reference electrode connected to both of the
sensing electrodes. In this case, one of the sensing electrodes and
its associated gas exposure chamber is used for the continuous
monitoring function, while the other is used only periodically,
under the control, again of microprocessor 66 for the purposes of
periodic recalibration. With this arrangement, a single electrolyte
chamber is used and provisions must be made to prevent or minimize
"cross-talk" between the electrodes via the electrolyte chamber in
such a system.
Yet another embodiment of the invention is illustrated in FIG. 6.
In this arrangement, a single sensor 100 is connected in series
with a mass flow meter 101 to pump 102 via valve 104. To measure
the zero background signal, valve 104 connects a by-pass air inlet
105 to pump 102, while disconnecting the flow meter 101 from the
pump. Once the zero signal S.degree. has been measured and
recorded, valve 104 connects flow meter 101 and air inlet 105 to
pump 102, thereby effecting an intermediate rate of flow of the
monitored gas entering through sample inlet 103 into sensor 100.
After recording the signals from sensor 100 and flow meter 101 at
the intermediate flowrate, the microprocessor 106 causes valve 104
to close off the bypass inlet 105, thereby increasing the flow
through sensor 100 to the maximum rate. After recording the sensor
and flow meter signals at the maximum flowrate, the processor
computes from the recorded data a newly recalibrated sensor
response constant k.sub.u (corresponding to the usual flowrate) and
uses this constant together with the latest S.degree. value to
compute the concentration of the sensed gas as the sensor signals
are being recorded in the course of regular monitoring.
Thus, as will be apparent from the foregoing, there are provided,
in accordance herewith, methods and apparatus for continuously
monitoring and sensing of toxic gases in various environments with
the instruments provided being so constructed and maintained as to
continuously monitor without the need for periodic manual
readjustment and recalibration of the instrument. With the
arrangements herein, the instrument may be mounted and properly
instrumented and programmed to readjust itself on a periodic basis,
with the period of readjustment also being determined as desired
and according to the particular location so as to continuously
monitor for any dangerous toxic gases in the environment. In
addition, the instrument may be arranged to recalibrate itself to
accommodate such variables as degradation of the instrument itself
and changes in the conditions under which monitoring is taking
place at any one time. As will be appreciated, the elimination of
periodic manual adjustments reduces substantially the cost of
maintenance of such instruments, particularly in remote
locations.
While the methods and apparatus herein disclosed form preferred
embodiments of this invention, this invention is not limited to
these specific forms of method and apparatus, and changes can be
made herein without departing from the scope of the invention which
is defined in the appended claims. For example, other forms of
sensor cells may be chosen by practitioners in the art, such as
those referred to in the prior art patents noted above, so long as
provision is made for a procedure to be carried out under the
control of selected instrumentation for automatic recalibration of
the instrument on a periodic basis, as required under the
circumstances of the environment in which the instrument is being
used.
* * * * *